1. Field of the Invention
The present invention relates to a process for preparing a polyolefin. More particularly, it relates to an improved process for polymerizing an olefin to obtain a polymer having a wide molecular weight distribution by using a novel catalyst having a remarkably high catalytic activity.
2. Description of the Prior Art
Heretofore, it has been well-known that catalysts made up of combinations of the transition metal compounds of Groups IVa - VIa of the Periodic Table and organo-metallic compounds of Groups Ia - IIIa of the Periodic Table, are remarkably effective for polymerizing an olefin at relatively low temperatures under relatively low pressure. However, these catalysts are not satisfactory from an industrial point of view for a number of reasons.
If the activity of the catalyst used for preparing the polyolefin is sufficiently high, the necessity of removal of the catalyst following polymerization can be avoided. The industrial advantages of the elimination of this step are very significant.
The molecular weight distribution of the resulting polymer affects both the formability of the polymer and the appearance of the molded product. That is, polymers having a narrow molecular weight distribution are suitable for injection molding. On the other hand, polymers having a wide molecular weight distribution are suitable for extrusion molding. Accordingly, it is quite advantagous industrially, to be able to preselect the molecular weight distribution and to achieve that selected distribution by control techniques. From both these viewpoints, the Ziegler catalysts and the other above-mentioned catalyst systems are not always satisfactory.
Previously, the present inventors had disclosed a process for polymerizing an olefin using a catalyst of high catalytic activity. In that previous disclosure the catalyst was indicated as being a combination of an organoaluminum compound and eutectic crystals which are prepared by reducing a mixture of titanium tetrahalide and vanadyl oxyalcoholate with an organoaluminum compound and separating the eutectic crystals from the unreacted materials and the solvent. (Japanese Patent Publication No. 11269/1974). The polymers prepared by using this catalyst system, however, have a narrow molecular weight distribution and are not suitable for extrusion molding.
A need continues to exist for an industrially acceptable process for polymerizing an olefin whereby the achievable molecular weight distributions using a catalyst with very high activity, can be enlarged.